Alfred Wegener: The Father of Plate Tectonics

• Who was Alfred Wegener?
  German meteorologist and geophysicist who proposed the Continental Drift Theory in 1912.

• Continental Drift Theory
  Wegener suggested that continents were once part of a supercontinent called Pangaea and have since drifted apart.

• Evidence Supporting Continental Drift

  • Fossil Distribution: Similar fossil species found on continents now separated by oceans.
  • Rock Formations: Similar rock types and mountain ranges on now-separated continents.
  • Paleoclimate Indicators: Unusual climate evidence, like glacial deposits in tropical regions.

• Initial Reception:
  Skepticism due to lack of a plausible mechanism for continental movement.

Alfred Wegener a few years before his death in 1930. Source: Steven, E. (2015)

Global Geological Models of the Early 20th Century

• Static Earth Models:
  Early theories assumed Earth’s features were fixed and unchanging.

• Continental Movement Debates:
  Wegener’s theory sparked discussions about the possibility of continental and oceanic movement.

Prevailing Theories Before Plate Tectonics

1. Contractionism
   • Theory: Earth is slowly cooling and shrinking, causing the formation of mountain ranges like “wrinkles on a dried-up apple.”
   • Problems:
     • Insufficient cooling to cause necessary shrinking.
     • Conflicts with the Principle of Isostasy (prevents continents from sinking).
2. Permanentism and Geosyncline Theory
   • Permanentism: Belief that continents and oceans have remained generally the same.
   • Geosyncline Theory: Suggested that mountain chains formed from thick deposits of sediments along continental margins.
   • Geosynclines: Depressions filled with sediment, explaining sedimentary layering but lacking support for large-scale continental movement.

Continental Drift Theory

The distribution of several Permian terrestrial fossils that are present in various parts of continents that are now separated by oceans. Source: Steven, E. (2015)

• What is it?
  Wegener’s theory that continents were once joined as Pangaea and have drifted apart over millions of years.

• Key Evidence for Continental Drift:

  • Fossils: Identical fossils on continents separated by oceans (e.g., Mesosaurus).
  • Rock Formations: Similar rock types and mountain ranges on separated continents.
  • Paleoclimate Indicators: Evidence of past glacial activity in now-warm areas.
  • Fit of the Continents: The remarkable alignment of the eastern coastline of South America with the western coastline of Africa was one of Wegener’s primary observations supporting his hypothesis.
  • Geological Continuity: When the continents are “fit” back together, matching geological formations and fossil distributions across these boundaries make more sense, strengthening the case for their once-connected state.

Seafloor Spreading

• What is it?
  Theory that new oceanic crust forms at mid-ocean ridges and moves outward, driving the movement of tectonic plates.

• Key Concepts:

  • Mid-Ocean Ridges: Sites where magma rises and forms new crust.
  • Ocean Basin Expansion: New crust formation pushes older crust away from the ridge.

• Evidence for Seafloor Spreading:

  • Magnetic Stripes: Symmetrical patterns on either side of ridges recording magnetic field reversals.
  • Ocean Floor Age: Rocks get older as you move away from mid-ocean ridges.
  • Heat Flow: Higher at mid-ocean ridges, suggesting rising magma.

Pattern of sea-floor magnetism off of the west coast of British Columbia and Washington. Source: Steven, E. (2015)

Plate Tectonics Theory

• What is it?
  A theory that Earth’s lithosphere is divided into plates that move over the semi-fluid asthenosphere, shaping Earth’s surface.

• Key Components:

  • Lithospheric Plates: Large rigid plates made of oceanic or continental crust.
  • Plate Boundaries: Regions where plates interact (convergent, divergent, transform).
  • Mechanisms of Motion: Driven by mantle convection, slab pull, and ridge push.

• Evidence Supporting Plate Tectonics Theory:

  • Earthquake and Volcano Patterns: Concentrated along plate boundaries.
  • Fossils and Rock Formations: Support continental drift.
  • Direct GPS Measurements: Confirms movement rates and directions of plates.

The layers of the tectonic Earth, (Source: Jain, S. (2014))

Types of Plate Boundaries

• Convergent Plate Boundary:
  • Ocean–Ocean Convergence: Forms volcanic island arcs.
  • Ocean–Continent Convergence: Forms volcanic mountain chains.
  • Continent–Continent Collision: Creates mountain ranges (e.g., Himalayas).
• Divergent Plate Boundary:
  • Plates move apart, forming mid-ocean ridges or rift valleys.
• Transform Plate Boundary:
  • Plates slide past one another, causing earthquakes (e.g., San Andreas Fault).

The Ocean–Ocean convergence

The Ocean–Ocean convergence, (Source: Jain, S. (2014))

The Ocean–Continent convergence

The Continent–Continent Collision

Divergent Plate Boundary

The Divergent Plate Boundary, (Source: Jain, S. (2014))

Example of Divergent Plate Boundary

The East Africa Rift Valley and the Red Sea rift, (Source: Jain, S. (2014))

Hot Spots and Mantle Plumes

• What are they?
  Fixed zones of volcanic activity caused by mantle plumes (e.g., Hawaii).

• Significance in Plate Tectonics:

  • Form volcanic islands as plates move over the hot spots.
  • Provide clues about plate movement and mantle dynamics.

Schematic model for oceanic volcanism, Source: Winter, J. (2014)

Plate Tectonics and Volcanism

• Convergent Boundaries:
  Volcanism occurs when subducted material melts and rises to form volcanoes.

• Divergent Boundaries:
  Upwelling magma fills gaps, creating volcanic activity.

Plate Tectonics and Metamorphism

• Subduction Zones:
  High-pressure, low-temperature metamorphism occurs as plates descend.

• Collisional Boundaries:
  High-pressure and high-temperature conditions create metamorphic rocks during continent collisions.

Summary

• Continental Drift: First suggested by Wegener, with evidence from fossils, rocks, and climates.
• Seafloor Spreading: Provides mechanism for continental drift; supported by magnetic patterns and seafloor ages.
• Plate Tectonics Theory: Unifies both theories, explaining global geological processes, earthquake patterns, volcanic activity, and mountain formation.